Question

Three complexes,
\([\mathrm{CoCl(NH_3)_5}]^{2+}\) (I),
\([\mathrm{Co(NH_3)_5H_2O}]^{3+}\) (II) and
\([\mathrm{Co(NH_3)_6}]^{3+}\) (III)
absorb light in the visible region. The correct order of the wavelength of light absorbed by them is :

• A. (III) \(\gt\) (I) \(\gt\) (II)
• B. (III) \(\gt\) (II) \(\gt\) (I)
• C. (I) \(\gt\) (II) \(\gt\) (III)
• D. (II) \(\gt\) (I) \(\gt\) (III)

Hint:

Wavelength absorbed is inversely proportional to the crystal-field splitting; use the spectrochemical series (Cl- \(\lt\) H2O \(\lt\) NH3).

Answer 1

The Correct Option is C

Spectrochemical series: \(\mathrm{Cl^-} \lt \mathrm{H_2O} \lt \mathrm{NH_3}\) (field strength increases).
Crystal field splitting \(\Delta_o\): (I) with one Cl^- \(\lt\) (II) with one H2O \(\lt\) (III) with six NH3.
Absorbed wavelength \(\lambda = hc/\Delta_o\), so \(\lambda \propto 1/\Delta_o\).
Therefore \(\lambda\): (I) \(\gt\) (II) \(\gt\) (III).
Answer: (3) (I) > (II) > (III)